The present invention relates generally to lateral photodetectors and more particularly to a technique for improving the response time and sensitivity of such detectors. The detectors are used in guidance systems to measure the position and intensity of laser energy reflected from a target.
The typical lateral photodetector (LPD) consists of a silicon wafer on which transparent layer of gold is deposited. The deposited gold forms a metal-semiconductor, Schottky barrier on one face of the wafer. On the other face, four signal electrodes are attached to extract the four position-sensitive signals. The four electrodes measure the position and intensity of the laser energy reflected from the target. The linear output of the electrodes is a function of the laser pulse intensity focused on the target and the position of the target with reference to the optical axis of the detector. The linear seekers capable of following such a signal are more accurate than conventional bang-bang seekers presently employed.
The Schottky barrier on the cell is negatively biased during operation. The barrier has the property of allowing optically generated holes to flow out of the cell into the gold barrier, but preventing electrons from the gold from entering the cell. The application of the negative bias to the barrier causes the electrons in the silicon to move away from the barrier, forming a "depletion region," i.e., a region depleted of electrons. As more bias is applied, the thickness of this depleted region grows. The thickness of the region determines the capacitance of the cell; therefore, increasing the thickness of this region, either by increased voltage or by using higher-resistivity silicon, reduces the capacitance of the cell. A reduced capacitance gives a cell with faster response time. In usage, one desires an LPD with a fast response time and a large signal response to optical pulses of light, i.e., a high responsivity.